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Billc wrote: lightly tamped coffee allows more flow and the pressure is decreased, tightly tamped coffee restricts water more resulting in higher pressure

Surprised you say that, Bill. Many people have observed that, given an initial "preinfusion period," there is little difference in the average flow rate when heavily-tamped shots and lightly-tamped shots are compared.

For instance, at the 2006 SCAA show, Michael Teahan did a short presentation on his experiment comparing 10lb tamped shots to 250lb tamped shots. No difference in shot timing or average flow rate.


...split from Recommended FLOW for rotary espresso pump by moderator...

Andy,
I think you will be very surprised at the pressure difference. I too viewed a similar exercise and without a pressure gauge that measures pressure closer to the brew head (or after the restrictor) you will not see it. If you have a GS3 a cool exercise is to take out the flow restrictor from the tube in the group and then install it in the inlet tube to the boiler (lower right of the brew boiler). And I just did it here with a GS3. I did see a difference even with preinfusion. However it is close to midnight so I don't dare drink the stuff.

What was even more interesting is the ability to dial in your grind (using the same tamp) with this pressure measurement. Similarly, coarser grounds, larger flow rate, lower pressure and vice versa.

I am sure there is a some condition that can prove me wrong.

Bill

Billc wrote:I think you will be very surprised at the pressure difference....What was even more interesting is the ability to dial in your grind (using the same tamp) with this pressure measurement. Similarly, coarser grounds, larger flow rate, lower pressure and vice versa

I don't doubt for a minute that there normally is an inverse relationship between flow rate and pressure measured at the group. But...I have never been able to achieve a faster flow rate with a lighter tamp, or a slower flow rate with a heavier tamp.

I just tried it again this morning: two pairs of shots, same grind, same dose, varied only the tamp pressure. Ran the shots for 30 sec then cut off the pump and measured the mass of espresso. The heavier-tamped shots actually weighed more; IOW, heavier tamp = faster flow rate (in this little test). That's unusual, normally I see no significant flow rate difference between the two.

So I continue to be perplexed when people advise to lighten up on the tamp in order to increase flow rate. But hey, that's espresso. It's very hard to duplicate someone else's results on your equipment.

But...I have never been able to achieve a faster flow rate with a lighter tamp, or a slower flow rate with a heavier tamp.

You actually can do it. For the past couple months I've been experimenting with grind particle size as it relates to espresso taste(at least for me), and I was trying to keep timing the same, but up the size,(more course grinder setting). With the fineness setting I have it at now, it takes a tamp pressure of just about 830lbs. to get in the 28-30 sec. range.
I'll check tomorrow to see the difference in time, but I think it was more than 30 seconds difference at a standard tamp. Hydraulic press against aluminum plate to basket bottom.Yeah silly I know, but interesting

Jepy wrote:With the fineness setting I have it at now, it takes a tamp pressure of just about 830lbs. to get in the 28-30 sec. range.

Dood, you tryin' out for shot put in the 2012 Olympics?

This mornings shots came in at about 26 seconds apart for the same volume. Scale showed just over 2 grams difference.

Dood, you tryin' out for shot put in the 2012 Olympics?

Andy, you should know we here at HB only train for the Sprolympics

John

Is there a measurable difference at lower deltas?
Say 5lbs to 60lbs?

-c

I've been extremely sceptical of tamping pressure for a long time, and certainly within human ranges I don't see much of a difference over about 20lb.

However, I've seen shots slow waaaaay down when nutating quite aggressively in the tamp. Which kind of freaks me out.

This is a difficult one.

I am also skeptical about any beneficial influence of high tamping pressure and prefer a fine grind and light tamp, which works well with the big grinders and lever machines I use in terms of consistency as well as taste profile and mouthfeel.

However, before I discovered decent hand grinders, I used a La Pavoni and Italian pre-ground coffee blends while travelling. Tamping hard and packing the basket almost to the brim did then slow things sufficiently to get a passable extraction. But this was on reflection not just a matter of tamp but of dose as well. What's more, if opting for just one pull on a lever machine, it is also a matter of shot volume. And quite possibly preinfusion as well. So there are really at least three factors in play.....

I believe it's important to ask similar questions rather than assuming that one can change the tamp as a variable in isolation.